Procedure for the recovery of diolefins from a mixture thereof with monoolefins

ABSTRACT

A cyclic process is provided for the recovery of diolefins from vapor mixtures thereof with monoolefins by selective adsorption over a solid adsorbent. A mixture of diolefins and monoolefins is circulated in the presence of a solid adsorbent whereby the diolefins are selectively adsorbed. After the solid adsorbent has adsorbed the diolefins, it is treated with superheated steam to cause desorption of the diolefins and the production of a mixture of gaseous diolefins and water vapor. The vapor mixture is then condensed and the diolefins are separated from water by decantation. The adsorbent is dried and cooled by flushing with the mixture of monoolefins from which diolefins have been removed, to complete the regeneration of the adsorbent so that it is suitable for reuse.

United States Patent Inventor Pierre Dassese .Iemeppe-Sur-Sambre.Belgium Appl. No 9,559 7 Filed Feb. 9, 1970 Patented Aug. 3, 1971Assignee Solvay & Cie

Brussels, Belgium Priority Feb. 10, 1969 Belgium 69,856

PROCEDURE FOR THE RECOVERY OF DIOLEFINS FROM A MIXTURE THEREOF WITHMONOOLEFINS 8 Claims, 1 Drawing Fig.

3,186,144 6/1965 Dow 3,306,006 2/1967 Urban ABSTRACT: A cyclic processis provided for the recovery of diolefins from vapor mixtures thereofwith monoolefins by selective adsorption over a solid adsorbent. Amixture of diolefins and monoolefins is circulated in the presence of asolid adsorbent whereby the diolefins are selectively adsorbed. Afterthe solid adsorbent has adsorbed the diolefins, it is treated withsuperheated steam to cause desorption of the diolefins and theproduction of a mixture of gaseous diolefins and water vapor. The vapormixture is then condensed and the diolefins are separated from water bydecantation. The adsorbent is dried and cooled by flushing with themixture of monoolefins from which diolefins have been removed, tocomplete the regeneration of the adsorbent so that it is suitable forreuse.

PATENTED Am; 3191: 3.596136 PROCEDURE FOR THE RECOVERY OF DIOLEFINS FROMA MIXTURE THEREOF WITI-l MONOOLEFINS BACKGROUND OF THE INVENTION Thisinvention is directed to a-process for'the recovery of diolefins byselectiveabsorption, starting from vapor mixtures thereof withmonoolefins.

It is often difficult and costly to separate diolefins and monoolefinsby knowncmethodssuch as by distillation, condensation or extraction whenthese derivatives have physical and chemical properties which aresimilar.

This is particularly the case in the recovery of diolefins from vapormixtures thereof with monoolefins. Anexample is in the production ofrubbery copolymers of monoolefins and diolefins of which the betterknown are the ethylenepropylene-diene terpolymers. When polymerizationis terminated, the copolymer is separated from the unreacted monomers.The latter are in the form of a vapor mixturecomprising a highproportion of monoolefins and a low quantity of diolefins.

Although monoolefins are easily obtained by cracking from petroleumfractions, diolefins must be synthesized by means of rather complexprocesses. Since only a portion of the diolefins presentinthepolymerization medium is incorporated in the copolymer, theproduction of copolymers of monoolefms and diolefins is not economicallyfeasible if the diolefins are not totally recovered and recycled.

Until now, the problem involving the recovery of diolefins from vapormixtures thereof containing highproportions of monoolefms hasnotbeensolved satisfactorily stJMMAaY or THE INVENTION A process has now beenfound by means of which it is possible to completely recover diolefms inadmixture with a major a first stage of separating the diolefins fromthe monoolefins by circulating the mixture of diolefms and monooletinson a solid adsorbent whereby the diolefins are selectively adsorbed,

a second stage of separating the diolefins from the adsor bent bytreating the adsorbent containing the diolefins with superheated steam,collecting the mixture of 'diolefms and steam, condensing this mixtureand separating the diolefms from water by decantation, and

a third stage of drying and cooling the adsorbent which is now freed ofdioletins by circulating thereon the mixture of monoolefins which hasbeen separated from diolefins in the first stage. In this manner, theadsorbent is in a suitable state for reuse for the adsorption or firststage of the process.

The apparatus used to carry out this process which also forms part ofthe present invention includes solidadsorbent disposed ina fixed bed ina plurality=of enclosures means to contact the solid adsorbent with avapor mixture of monoolefin and diolefin to adsorb the diolefin, meansfor withdrawing the monoolefms from the enclosures, means for feedingsuperheated steam into contact with said adsorbent to separate theadsorbed diolefins, means for removing a vapor mixture of steam anddiolefin from said enclosure, decanting means to separate the mixture ofdiolefin and water, means for contacting said adsorbent withtheseparated monoolefins to dry said adsorbent and means for withdrawingfrom said en closures a mixture of steam and monoolefins and forseparating the monoolefins therefrom.

BRIEF DESCRIPTION OF THE DRAWING In the drawing which illustrates oneembodiment of the invention, the FIGURE is a schematic illustration ofone type of DESCRIPTIONOF THE PREFERRED EMBODIMENTS The process of theinvention is applied to mixtures contain ing one or more monooleflns aswell as one or more diolefins in the vapor phase. The present process isespecially useful for treating mixtures of monoolefins and diolefinsobtained when separating unconverted monomers during the production ofrubbery copolymers by polymerization in the absence of solvent, whilethe copolymers are dispersed in the monomers maintained in the liquidstate. Such apolymerization can be carried out very easily and with aparticularly high diolefm conversion rate, by using the supportedcatalysts which have been described in U.S, Pat. No. 3,396,l55 assignedto Solvay & Cie.

The mixtures obtained comprise one or more lower aolefins, i.e.,a-olefins containing up to about 6 carbon atoms such as ethylene,propylene, butened, isobutene, hexene-l or 4-methylpentene-l and a smallproportion of a diolefin. The diolefins may be conjugated ornonconjugated, linear or cyclic, such as butadiene, pentadiene,hexadiene or dicyclopentadiene. Generally the diolefins used containfrom 4 to about l6 carbon atoms, and for purposes of polymerization, thelinear diolefins usually contain a-unsaturation. However, thepresent-process can be applied to vapor phase mixtures of monoolefinsand diolefins which areobtained from any source.

The process according to the invention is especially suitable for therecovery of diolefinsin which the physical properties, particularly thevapor tension under normal pressure, are generally in the same range asthose of the monoolefins from which they must be separated. While thepresent process can be used to separate mixtures of monoolefins anddiolefins in any proportion, it is especially economical and hencedesirable when the diolefin content of the mixture is lower than that ofthe monoolefin, i.e. below SO'percent molar. It is particularly usefulfor separating mixtures of monoolefins and diolefins containing lessthan 20 percent molar and preferably less than 10 percent molar ofdiolefins. For example, in the production of terpolymers of ethylene,propylene and diene, the mixture I of unreacted monomers generallycontains a major portion of propylene and contains only a few molarpercent of ethylene and diene. when the diene is selected from therelatively low molecular weight dienes, such as butadiene, pentadiene orhexadiene, it is practically impossible to make a complete recovery by aprocess other than the one according to the invention.

The solid materials used as adsorbent may be selected among thosegenerally used for this purpose, such as those set forth in Perry'sChemical Engineers Handbook, McGraw Hill Book Co., 4th Edition, withparticular reference to pp. 16-2 and l6-5; which pages are incorporatedby reference.

The solid materials which are generally used with most advantage arethose of the activated charcoal type. These materials are usually in theform of porous particles having a high specific surface as is more fullydescribed in the above noted reference. They are used according to knownmethods such as in a fixed bed, movable bed or fluidized bed.

The fixed bed method is preferred due to its simplicity. The fixed bedmethod is carried out batchwise and a plurality of adsorption beds aregenerally employed.

The process according to the invention may also be carried outcontinuously by using the methods known as movable bed or fluidized bed.The various adsorption and desorption stages are then carried out indifferent apparatuses.

Whatever the method used, the process of the invention comprises asequence of adsorption and desorption stages which form a closed cyclefor the treatment of the solid adsorbent.

The first stage comprises the treatment of the solid adsorbent with themixture of diolefins and monoolel'ms. lt is during this stage that thediolefins are selectively adsorbed on the solid adsorbent. At the startof the diollefin adsorption stage,

. the solid adsorbent is preferably at a-temperature lower than comparedto the total fluid flow. The pressure of the mixture may be raised abovenormal pressure if desired in order to influence favorably theadsorption of the diolefins.

When operating according to one of the continuous methods, a fraction ofthe solid adsorbent is continuously withdrawn and transferred to thedevices where regeneration is carried out and a portion of solidadsorbent which is saturated with monoolefins is continuouslyreintroduced.

When operating batchwise, the fixed bed may be'flushed with the mixtureof diolefins and monoolefins until the presence of diolefins can bedetected in the output mixture. The fixed bed is then submitted to thenext regeneration stage and the mixture of diolefins and monoolefins iscirculated in contact with'another fixed bed.

The switching may be controlled automatically by a detector mountedunder the output duct. The detector may for ex- .ample be achromatographer used to make automatic samplings at specific timeintervals.

The second stage comprises the regeneration of the solid adsorbent bymeans of superheated steam, and the desorption of the diolefins. Thesuperheated steam displaces the diolefins which are desorbed and arecarried away as a vapor mixture with water vapor. The latter mixture isthen condensed in a heat exchanger and decanted in order to recover thediolefins and water separately. In most applications. the mixturecontains only one diolefin which, after having been dried for example byadsorption on a'molecul'ar sieve, may again be used directly. When manydiolefins are used for the polymerization, the diolefins which arerecovered after drying may be recycled This stage also comprises thecooling of the solid adsorbent and saturation thereof with monoolefin.When drying is terminated, the adsorbent is saturated with monoolefinsat a temperature which is higher than the one at which the adsorption ofthe diolefins iscarried out. The solid adsorbent is cooled by flushingwith the mixture of monoolefins free of diolefins. This mixture may becollected at the exit of the first diolefins adsorption state. The endof this stage is determined by measuring the temperature of the exitgases.

Usually the drying and cooling stage are combined. in this case, themonoolefin gaseous mixture which exits from the first stage is directlyfed at the input of the water-saturated by reestablishing suitableconcentrations and this can be carried out by bringing a supplementaryquantity of the diolefin the consumption of which has proportionatelybeen higher.

The mixture of diolefins and water vapor also contains a smallproportion of monoolefins which corresponds to the equilibrium ofadsorption during the first stage. These monoolefins are not condensedin the heat exchanger and form a gaseous phase in the decanter. Toprevent a loss of the quantity of diolefins present in this gaseousphase, it is possible to feed back this gaseous phase with a new supplyof diolefins and monoolefins to the adsorption stage.

The temperature of the steam used during regeneration is not criticaland generally the superheated steam available in the plant is used. Thesteam should be sufficiently overheated for example, about 2 or 3 C.above the saturation temperature to prevent any water condensation onthe solid adsorbent. Heat insulation of the apparatuses in which thisstage is carried out decreases the heat waste and'enables the use ofwater vapor the latter having lower enthalpy.

When operating batchwise, the treatment of the solid adsorbent withsuperheated steam may be stopped when there are no more traces ofdiolefins in the water vapor which is removed from the fixed bed.Detection and commutation towards the next stage may also be automatic.

. The third stage comprises the drying of the solid adsorbent. At theend of the second stage, the solid adsorbent is saturated with waterat atemperature which is near that of superheated vapor (more than 100 C. ifthe secondstage is carried out under atmospheric pressure). Drying iscarried out by mere flushing with the mixture of monoolefins the latterbeing free of diolefins. This mixture may be sampled at the exit of thefirst stage where the diolefins are adsorbed. Drying of the solidprevents a decrease of the adsorption capacity, and it comes to an endwhen water vapor is no longer detected at the exit.

fixed bed which has undergone the second regeneration stage. At thestart, flushing with the mixture of monoolefins assures the progressivedrying of the bed starting from the entrance. Then cooling is carriedout progressively.

When operating batchwise, the first and third stage may be combined andcarried out in series without having to store, warm "or cool the gaseousmixture between each passage over a fixed bed. in this combination, thefirst stage of adsorption of the diolefins is carried out in one or morefixed beds of solid adsorbent arranged in series: The gaseous mixturewhich exits, then comprises monoolefins only and is at a temperaturelower than 50 C., preferably near room temperature. This mixture may bedirectly introduced at the entrance of the fixed bed(s) where coolingand drying of the solid adsorbent are carried out.

However, the stages of cooling and drying of the adsorbent may also becarried out separately. in this case, the gaseous mixture which exitsfrom the bed(s) in the adsorption stage is introduced in the bed(s) inthe cooling phase.

The mixture which exits from the first stage is slightly warm, i.e.usually between 50 C. and ambient temperature. It is directly introducedin the fixed bed(s) where drying of the solid adsorbent is carried out.The mixture which exits comprises water vapor which is extracted bypassing the same through a condenser. Independently, one or more fixedbeds are treated with superheated steam in order to carry out the secondstage of regeneration of the solid adsorbent.

When the three stages last about the same period of time, it issufficient to have three fixed beds of solid adsorbent to carry out theprocess according to the invention in a batchwise manner. The switchingsare carried out simultaneously and can be controlled for example by adiolefin detector connected to the exit of the bed where the first stageis carried out.

When the various stages are of unequal durations, each may be carriedout with beds having the same volume, mounted in series. The number ofbeds is proportional to the duration of the stage, and the switchingshould be programmed as a function of the duration of each stage. It isrecommended that the end of each stage be automatically detected or thatthese various means be combined When operating continuously, each phaseis carried out in a distinct apparatus or compartment containing avolume of solid adsorbent which is proportional to the duration of thestage. The solid adsorbent circulates continuously between each of theseapparatuses or compartments.

The example which follows will serve to provide a better understandingof the advantages of the invention. This example illustrates the bestmode currently contemplated for carrying out the present invention andmust not be construed as limiting the scope thereof in any manner. 1

EXAMPLE The process of the invention is used for the recovery oftrans-l,4-hexadiene starting from a gaseous mixture in which it ishighly diluted with propylene and ethylene. This mixture originates froma plant for the production of terpolymers of percent oftrans-1,4-hexadiene. The flow and the composition may vary slightly,depending on the polymerization conditions. Thetemperature of themixture is about 15 C. and the absolute pressure is about 1.5 kg./em.abs.

The solid adsorbent is an activated charcoal known under the trademarkSorbonorit" and produced by Societe Norit (Netherlands). The process iscarried out by means of a batchwise fixed bed method.

With reference to the figures, three fixed beds 1, 2 and 3 eachcontaining 50 kg. of activated charcoal are used. The gaseous mixture ofpropylene, ethylene and trans-l,4-hexadiene is introduced by means ofpipe duct 4. The apparatus is supplied with superheated water vapor(absolute pressure of 1.05 kg./cm. and temperature of 130 C.) by meansof pipe duct 5. i

The apparatus also comprises a duct 6 for transferring the mixture ofpropylene and ethylene. An automatic chromatographer 7 which analyzesthe mixture of propylene and ethylene at every half hour is connected tothis duct. When a peak characteristic of trans-1,4-hexadiene isregistered on the chromatographer, the fixed beds are switched toanother bed since the diene is no longer being adsorbed in the originalbed.

Duct 8 is used to remove the mixture of water vapor andtrans-1,4hexadiene. This mixture is cooled in the heat exchanger 9 whereit iscondensed; water at a temperature of about 4 C. is used as thecooling means. The condensate is poured off in the decanter 10; About1.8 liter/hour of liquid trans-1,4-hexadiene are collected at atemperatureof about l5 C. It is removed throughpipeduct 11. After havingbeen dried, it is recycled towards the polymerization apparatuses. Wateris poured off and it is extracted through the bottom of the decanter viaduct 12. The decanter also comprises a gaseous phase comprisingtrans-1,4-hexadiene, propylene and ethylene. in order to prevent anyloss of trans-1,4-hexadiene,

the top portion of the decanteris connected with the input of theapparatus via duct 13. i

The duct 14 is used to remove propylene and ethylene which may be mixedwith water vapor. This water is condensed in the heat exchanger 15 whichis cooled to room temperature by means of water. The separator 16 isused to remove water through duct 18 and to collect the mixture ofpropylene and ethylene by means of duct 17. This mixture is alsorecycled towards the polymerization apparatuses.

The three fixed beds 1,2 and 3 are successively submitted to the stagesdefining a complete cycle of adsorption and desorption oftrans-1,4-hexadiene; For example, when a new cycle begins for bed 1, atthe start of the cycle, the activated charcoal from which bed 1 is made,is saturated with propylene and ethylene at a temperature substantiallyequal to room temperature. Valves 19, 20, 23, and 24 are closed. The

valves 21 and 22 are open to permit the entry in the fixed bed 1 of themixture of propylene, ethylene and trans-1,4-hexadiene and the exit ofthe mixture of propylene and ethylene. This exit mixture issubstantially at the same temperature as the entrance mixture, i.e. l5C. As the mixture passes through the bed, transl ,4-hexadiene displacespropylene and ethylene in the adsorption stage. When a new cyclebeginsfor bed I,

the second stage begins for bed 2. Valves 26,27, 28 and '29 are closedand valves and 30 are open to permit a supply of overheated steam fromconduit 5 to pass through bed 2 and the removal of, water and desorbedtrans-1,4-hexadiene through conduit 8. Since the enclosures containingbeds 1, 2 and 3 are carefully heat insulated, the temperature of theoutput mixture is higher than 100 C. which prevents all watercondensation.

.During this time, the third stage, i.e. preparing the adsorbent forreuse, is carried out in bed 3. For this operation, valves 31, 33, 34and 36 are closed and valves 32, and 37 are open to permit flushing ofthe bed 3 with the mixture of ethylene and'propylene which exits frombed 1 and is transported through conduit 6. Flushing results in thedesorption of water which may still be present in bed 3 and of coolingthe same to a temperature close to room temperature. T he mixture whichexits from bed 3 contains water vapor which is conden'sed, separated andremoved through the openvalve 35 and through duct 14,

After about 4 hours, trans-1,4-hexadiene is no longer found in decanter10 which means that. desorption of trans-l ,4-hexadiene from bed 2 iscomplete. Valves 30 and 25 are then closed, as well as valves 35 and 37,and valv,es-..34, 32, 26 and 29 are opened. in this way the monoolefinexiting from bed 1 isstill circulated directly through bed 3, althoughin the opposite direction and in addition the warm mixture which exitsfrom the bed 3 is thencirculated, through the wet bed 2.!0 initiate thedrying thereof. The mixture of propylene, ethylene and water vapor frombed 2 exits by way ofvalve 29 andiscarri ed to heat exchanger 15 by wayofconduit l4. Iii bed 1 ad sorbing of trans-1,4-hexadiene is continuedby passage of the mixture of propylene, ethylene and trans-l,4-hexadiene therethrough. Cooling down of bed 3is also carried on bymeans of the mixture of propylene and ethylene which exits from bed 1.

After about 10 hours, the chromatograms which were periodically recordedby thechromatographer 7 indicate that bed lis saturated withtrans-1,4-hexadiene. The trans-1,4- hexadiene which escapes from bed 1at the end of the adsorption stage is adsorbed on the bed 3 and istherefore not lost. At this moment, flushing of the bed 1 is stopped andthe mixture of propylene, ethylene and trans-1,4-hexadiene is suppliedto bed3JFor this purpose, valves 21, 22, 26 and 29 are closed and valves33, 37, 24 and 19 are opened. In this manner, bed 3 is used instead ofbed 1, bed 2 is dried with the effluent from bed 3 and bed 1 is treatedwith'superheated steam. After l4 hours from the start of the operation,all of the diene has been stripped from bed 1 and therefore treatmentwith superheated steam is stopped and the mixture which exits from bed 2is passed through bed 1. After 20'ho urs, bed 3 is saturated withtrans-1,4-hexadiene. The adsorption of the latter compound on bed 2 iscarried out and bed 3 isthen regenerated. Finally, after 30 hours, bed Iis ready to start a new cycle.

The quantity of trans-1,4-hexadiene recovered in the dc canter 10represents morethan 99 percent of the quantity present in the startingmixture. The quantity of superheated steam used during each regenerationstage is about kg.

It will be realized that the process of the invention is particularlysuitable to recover nearly all the trans-1,4-hexadien which is dilutedas a vapor in admixture with propylene and ethylene and this can becarried out with a low energy output. As a matterof fact, no warming orcooling of the gaseous mixtures should be carried out before passing thesame through the fixed beds. The devices are simple, and comprise nomovable parts and the operations are not complicated. Furthermore, thereis no degradation of trans-l ,4-hexadiene whatever the number ofpassages through the apparatuses.

What l desire to claim and secure by Letters Patent is:

1. Cyclic process for the recovery of diolefins from vapor mixturesthereof with monoolefins by selective adsorption over a solid adsorbent,which comprises the following successive stages,

1. circulating a mixture of diolefin and monoolefin in contact with asolidadsorbent thereby to adsorb the diolefins;

2. treating the solid adsorbent containing adsorbed diolefins withsuperheated steam to separate the diolefins from said adsorbent,condensing the thus-produced vapor mixture of water and diolefins andseparating the diolefins and water by decantation, and

3. drying and cooling the adsorbent by flushing with the 'of the fixedbed is carried out, the bed in stage I being replaced by the bed instage 3 and the bed in stage I is used for stage 2.

8. Apparatus for recovery of a diolefin from vapor mixtures thereof witha monoolet'in by selective adsorption on a solid adsorbent comprisingsolid adsorbent disposed as a fixed bed in a plurality of enclosures,means to contact said vapor mixture with said solid adsorbent in orderto adsorb said diolefin and "thereby separate said diolefin from saidmonooletin, means to withdraw said monoolefin from said enclosures,means to contact said solid adsorbent with superheated steam in order toseparate said diolefin from said adsorbent, means for withdrawing amixture of diolefin and steam from said enclosures, decanting meansconnected to said means for withdrawing the mixture of diolefin andsteam to separate said diolefin and water, means connected to said meansfor withdrawing monoolefin to contact said solid adsorbent with saidmonoolefin in order to remove said steam from said adsorbent and meansconnected to said enclosures to remove a mixture of steam and monoolefinand to separate said monoolefin from said mixture of said steam and saidmonoolefin.

2. treating the solid adsorbent containing adsorbed diolefins withsuperheated steam to separate the diolefins from said adsorbent,condensing the thus-produced vapor mixture of water and diolefins andseparating the diolefins and water by decantation, and
 2. Processaccording to claim 1, in which the solid adsorbent is activatedcharcoal.
 3. drying and cooling the adsorbent by flushing with themixture of monoolefins from which the diolefins have been removed sothat the adsorbent is suitable for reuse in stage
 1. 3. Processaccording to claim 1, in which the solid adsorbent is used in the formof granules disposed as a fixed bed.
 4. Process according to claim 1, inwhich the solid adsorbent is used in the form of granules disposed as amovable bed.
 5. Process according to claim 1, in which the solidadsorbent is used in the form of granules disposed as a fluidized bed.6. Process according to claim 1, which comprises the successivebatchwise treatments of the solid adsorbent disposed as a fixed bed. 7.Process according to claim 6, in which the treatment of the solidadsorbent by the mixture of diolefins and monoolefins is automaticallystopped when the presence of diolefins in the mixture of monoolefinswhich exits from the fixed bed is detected and that at this moment, apermutation of the fixed bed is carried out, the bed in stage 1 beingreplaced by the bed in stage 3 and the bed in stage 1 is used for stage2.
 8. Apparatus for recovery of a diolefin from vapor mixtures thereofwith a monoolefin by selective adsorption on a solid adsorbentcomprising solid adsorbent disposed as a fixed bed in a plurality ofenclosures, means to contact said vapor mixture with said solidadsorbent in order to adsorb said diolefin and thereby separate saiddiolefin from said monoolefin, means to withdraw said monoolefin fromsaid enclosures, means to contact said solid adsorbent with superheatedsteam in order to separate said diolefin from said adsorbent, means forwithdrawing a mixture of diolefin and steam from said enclosures,decanting means connected to said means for withdrawing the mixture ofdiolefin and steam to separate said diolefin and water, means connectedto said means for withdrawing monoolefin to contact said solid adsorbentwith said monoolefin in order to remove said steam from said adsorbentand means connected to said enclosures to remove a mixture of steam andmonoolefin and to separate said monoolefin from said mixture of saidsteam and said monoolefin.